The present invention relates to golf ball materials which have a good thermal stability, flow and processability and from which there can be obtained high-performance golf balls endowed with excellent properties such as rebound, durability and softness. The invention also relates to golf balls which include as a component therein a molding made from such a golf ball material, and to methods for preparing such golf ball materials.
In recent years, ionomeric resins have been widely used as cover materials for golf balls. Ionomeric resins are ionic copolymers of an olefin such as ethylene with an unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic acid, in which some of the acidic groups are neutralized with metal ions such as sodium, lithium, zinc or magnesium. These resins provide excellent characteristics in terms of durability, rebound resilience and scuff resistance of the ball.
At present, the base resins used in golf ball cover materials are generally ionomeric resins, but a variety of modifications are being made to address the constant desire by players for golf balls having a suitable degree of flexibility, a high rebound resilience and an excellent flight performance.
For example, to improve the rebound resilience and the cost characteristics of ionomer cover materials, U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760 and International Application WO 98/46671 describe cover materials composed of an ionomeric resin to which a large amount of a metallic soap has been added.
However, the metallic soap in these cover materials undergo decomposition and vaporization during injection molding, generating a large amount of fatty acid gases. As a result, molding defects tend to arise. Moreover, the gases that have formed deposit on the surface of the molding, markedly lowering its paintability. The rebound resilience characteristics obtained with such cover materials do not differ much from those provided by ionomer covers of the same hardness which contain no metallic soap; the rebound resilience in both cases is about the same. Hence, only a small positive effect is observable from the inclusion of a metallic soap; such addition does not markedly increase the rebound resilience. Moreover, depending on the type of metallic soap used, the processability and the rebound resilience characteristics are sometimes significantly deteriorated (or declined), making the cover material entirely unfit for practical use.
In addition, materials in the form of simple molten mixtures of a soft thermoplastic resin and an ionomer have been developed (JP-A 2003-180878). However, even though the material appears to be uniform during the production process, there has been some concern that when the material is injection molded to form a golf ball layer, the high shear forces during molding will subject the layer formed by the molten mixture to delamination.
An ionomer for use as a golf ball material which has recently been developed is a homogeneous-phase, high-rebound resilience material that has an interpenetrating polymer network (IPN) structure (U.S. Patent Published Application No. 2004/0044136). The ionomer is obtained by mixing a first ingredient such as an ethylene-(meth)acrylic acid copolymer with a second ingredient that is a different type of thermoplastic resin to form a resin composition, then adding a metal ionic species as a third ingredient to neutralize the acids on the first ingredient dispersed in the resin composition. However, in this production method, if the different type of thermoplastic resin used as the second ingredient is poorly compatible or completely incompatible with the ionomeric resin matrix thereby created, when injection molding is carried out using this material to form a golf ball layer, the high shear forces in the mold sometimes cause delamination to occur within the layer, raising concerns over a decline in the properties of the golf ball. This tendency is most clearly observed in thermoplastic resins which have a poor heat resistance and are thus prone to thermal degradation.